SIL 2 1oo2

[fornhamspark]

Could someone please put me right here on the use of analogue signals.

Statement for an IDC book on Saftey Instrumented Systems

"1oo2 is dual channel which has a fault tolerance value of 1 because if one channel has a dangerous hidden fault, protection is still available via the redundant channel"

My question is one channel is needed to trip system, if one channel becomes faulty will this trip the system. Or if the channel becomes faulty do not trip system and inform hmi of problem.

 

[cjw32]

The IDC manual is correct because it specifies that it's dealing with a "dangerous hidden fault" which in standard terminology is dangerous unrevealed failure.

A dangerous unrevealed failure will not be detectable by the diagnostics but the safety function is still valid as the trip function will be implemented if only 1 out of the 2 channels trip (assuming the other channel is still operational).

Not all (channels) instruments initiate a trip signal on fault detection. It depends on the application and the equipment. Certain instruments I use provide alarm and fault contacts, thus allowing discrimination. We sometimes wire the fault and the alarm contact into the trip if it's a 1oo1 system. Some instruments output an alarm and fault contact combined, so there isn't much choice.

If it's an analogue signal then we use trip amplifiers to generate trip signals. The trip amplifiers can be specified to fail up scale or down scale etc depending on the application.

So to answer your questions. You are correct that one channel is needed to trip the system in a 1oo2 logic, but it does not follow that a fault will cause a trip because it is dependent on the system configuration. And it is possible on certain systems to configure the HMI to report detected faults. However, the IDC book is talking about unrevealed faults which would not be detected by the diagnostics. These would normally be revealed during the proof testing of the system, which is why the setting of the proof test interval is very important if you have a number of unrevealed dangerous failures.

 

[fornhamspark]

cjw32

Thankyou so much for replying, it seems that this subject is causing many people to bury their head in the sand.